Yeah, you need to Beat the E4 and the Poketransfer lab is just west of White Forest. There aren't any restrictions other than the transferred pokemon can't have any HM moves, and any items they have will be put in your bag automatically. You have to play a mini-game to transfer them, and any that don't get caught in the mini game stay on their original game. Luckily, you can always try again since, unlike with Gen 3-4, there's no limit on how many times you can transfer in a day.